1. Field of the Invention
The present invention relates to a semiconductor substrate having a superconducting thin film thereon and a process for producing the same.
More particularly, it relates to a single crystal semiconductor substrate having a superconducting thin film which is uniform in composition and possesses a high critical temperature (Tc) and a high critical current density (Jc) and a process for producing the same.
The superconducting thin film layer deposited on the semiconductor substrate according to the present invention can be used as a wiring material for integrated circuits which are built in the semiconductor substrate and for Josephson junctions in Josephson devices.
The semiconductor substrate according to the present invention can be used as a material for fabricating a novel circuit element which utilizes "an aproximity A proximity effect" between a semiconductor and a superconductor such as a superconducting transistor or a hot-electron transistor.
2. Description of the Related Art
The superconductivity is a phenomenon which is explained to be a kind of phase change of electrons under which the electric resistance becomes zero and perfect diamagnetism is observed. Several superconducting devices have been proposed and developed in electronics which is a typical field to which the superconducting phenomenon is applicable.
A typical application of the superconductor is a Josephson device in which quantum efficiency is observed macroscopically when an electric current is passed through a weak junction arranged between two superconductors. The tunnel junction type Josephson device is expected to be a high-speed and low-power consuming switching device owing to the smaller energy gap of superconducting material. It is also expected to utilize the Josephson device as a high-sensitive sensor or detector for sensing very weak magnetic field, microwave, radiant ray or the like since variation of electromagnetic wave or magnetic field is reflected in variation of Josephson effect and can be observed as a precise quantum phenomenon.
Development of the superconducting devices such as high-speed logic units or no power-loss wiring materials is also demanded in the field of high-speed computers in which the power consumption per unit area is reaching the upper limit of the cooling capacity with increment of the integration density in order to reduce energy consumption. In fact, one of the basic technologies for fabricating an integrated circuit on a silicon single crystal substrate is a metallization technology for forming patterned conductor lines which are used for interconnecting a variety of electronic elements which are fabricated by other basic technology including an insulating layer forming stage, a patterning stage of the insulating layer and an impurity doping stage by means of thermal diffusion, ion implantation or the like. However, a part of the signal current passing through the conventional metallic conductor lines is lost because the cross sectional area of the metallic wiring line is very fine in the case of integrated circuits which are built on silicon substrates. Therefore, it is necessary to deliver the signal current without current loss by means of a superconductor.
The critical temperature "Tc" of superconductivity, however, did not exceed the 23.2K of Nb.sub.3 Ge which was the highest Tc for the past ten years, so that such materials have not been used as wiring material for ICs.
The possibility of an existence of a new type of superconducting material having much higher Tc was revealed by Bednorz and Muller, who discovered a new oxide type superconductor in 1986 (Z. Phys. B64, 1986 p189).
It had been known that certain ceramic materials of compound oxides exhibit the property of superconductivity. For example, U.S. Pat. No. 3,932,315 discloses Ba-Pb-Bi type compound oxide which shows superconductivity and Japanese patent laid-open No. 60-173,885 discloses that Ba-Bi-Pb type compound oxides also show superconductivity. These superconductors, however, possess rather lower transition temperatures of about 10K and hence usage of liquidized helium (boiling point of 4.2K) as cryogen is indispensable to realize superconductivity.
The new type compound oxide superconductor discovered by Bednorz and Muller is represented by [La, Sr].sub.2 CuO.sub.4 which is called the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxides. The K.sub.2 NiF.sub.4 -type compound oxides show such higher as 30K which are extremely higher than known superconducting materials.
It was also reported that C. W. Chu et al. discovered, in the United States of America, another superconducting material so called YBCO type represented by YBa.sub.2 Cu.sub.3 O.sub.7-x having the critical temperature of about 90K in February 1987 (Physical Review letters, Vol. 58, No. 9, p908).
The other type new superconducting materials which were reported recently are a compound oxide of Bi-Sr-Ca-Cu-O system reported by Maeda et al (Japanese journal of Applied Physics. Vol. 27, No. 2, p 1209 to 1210) and Tl-Ba-Ca-Cu-O system which exhibit such high Tc as more than 100K (Hermann et al. Appl. Phys. Lett. 52 (20) p1738) and which are chemically much stable than the abovementioned YBCO type compound oxide or the like. And hence, the possibility of an actual utilization of the high Tc superconductors has burst onto the scene.
The above-mentioned new type superconducting materials can be deposited on a substrate in a form of a thin film by physical vapour deposition (PVD) technique or chemical vapor deposition (CVD) technique.
In the conventional process for preparing such thin film of superconductors on substrates, a vapour source or a target of an oxide which is prepared by sintering technique is vaporized or sputtered to deposit a thin film of superconductor which is then subjected to an after-treatment in which the thin film deposited is heated or annealed in an oxygen-containing atmosphere or which is exposed to oxygen plasma. Such after-treatment is indispensable to realize the desired superconducting properties because the superconducting properties of the above-mentioned new type compound oxide superconductors are influenced by the oxygen deficiency in the crystal. In fact, if the oxygen deficiency in the crystal is not proper, Tc can not become higher and the discrepancy between the onset temperature and a temperature where perfect zero resistance is observed becomes large.
The conventional sputtering technique for preparing a thin film composed of the ceramics type oxide superconductors represented by the formula BaPb.sub.1-x Bi.sub.x O.sub.3 (in which 0.05.ltoreq..times..ltoreq.0.35) is disclosed in Japanese patent laid-open No. 56-10,9824. In this patent, the superconducting thin film which is represented in an oxygen-containing atmosphere by high-frequency sputtering technique while the substrate is heated in order to improve ordering or orientation of the crystal. The resulting film deposited on the substrate is further heat-treated at 500.degree. to 550.degree. C. in order to increase the oxygen-content in the thin film but the reference mentions nothing about the conditions required how to prepare the thin film of high-Tc superconductor which discovered after the filing date of this patent.
Still more, the substrates on which the thin films of the new type superconductors are deposited are limited to very special materials such as single crystal oxides of MgO, SrTiO.sub.3 or the like because the substrates are required to have the same or similar lattice constants as the superconducting thin film to be deposited in order to realize desired crystalline structures of superconductors. Since the single crystal oxides such as MgO or SrTiO.sub.3 possess similar lattice constants to the new type superconductor and are chemically stable, they have been usually used as the substrate. However, they are not semiconductors but insulators.
In order to realize the above-mentioned new type circuit element which utilizes "an A proximity effect" between a semiconductor and a superconductor such as a superconducting transistor or a hot-electron transistor, it is requested to deposit the thin film of superconductor on the semiconductor, particularly on a silicon single crystal which is predominantly used in the IC industry. An idea of such new devices such as a superconducting transistor or a hot electron transistor have been proposed (M. Heiblum et al. "Solid State Electronics" Vol. 24, No. 343-346, 1981) but have not yet realized as an actual device by utilizing the oxide type superconductors. In order to realize the superconducting transistor, hot electron transistor or FET, it is indispensable to prepare such a semiconductor substrate that has a superconducting layer deposited homogeneously thereon. The conventional metal type superconductors such as Nb.sub.3 Ge are difficult to be deposited uniformly on the semiconductor and have very lower critical temperatures.
It is proposed to replace the insulator substrates such as MgO or SrTiO.sub.3 by a semiconductor substrate so that a superconducting thin film is deposited directly on the semiconductor substate by the conventional sputtering technique. In this case, however, it is very difficult to produce reproductively the superconducting thin film having desired quality.
The reasons that a homogeneous superconducting layer can not be formed on a semiconductor substrate are thought as follow:
Firstly, the substrate on which the superconducting thin film is deposited must be heated at a high temperature of about 700.degree. C. during the sputtering operation. Therefore, when a semiconductor substrate such as a silicon substrate is used, an element of the substrate such as silicon diffuses into the superconducting thin film layer, resulting in deterioration of superconducting property. Secondly, the desired crystalline structure of superconductor can not be realized in the thin film because of mismatch of lattice constants between the superconductor and the semiconductor. Finally, fine cracks are produced in the superconducting thin film deposited because of the difference in thermal expansion coefficients between the superconductor and the semiconductor.
The present applicant already proposed several processes for preparing the thin films of the high-Tc superconductor on oxide substrate in the following patent applications: U.S. patent application Ser. No. 152,714 filed on Feb. 2, 1988, U.S. patent application Ser. No. 167,895 filed on Mar. 13, 1988, U.S. patent application Ser. No. 195,145 filed on May 18, 1988, U.S. patent application Ser. No. 195,147 filed on May 18, 1988, U.S. patent application Ser. No. 200,206 filed on May 31, 1988, U.S. patent application Ser. No. 286,860 filed on Dec. 20, 1988, U.S. patent application Ser. No. 289,718 filed on Dec. 25, 1988, U.S. patent application Ser. No. 289,719 filed on Dec. 25, 1988, U.S. patent application Ser. No. 290,309 filed on Dec. 26, 1988 or the like. The present invention is completed on the same line of these patent applications.
An object of the present invention is to provide a semiconductor substrate having a superconducting thin film thereon and a process for producing the same.